combobox: fix picker each key

key was formed by `i + option.value`, which if the option value was in
the form of a number would end up as addition instead of concatenation.
replaced this key with just `option.value`, as this field is required.

src/lib/Combobox.svelte

@@ -396,7 +396,7 @@
 			}}
 			tabindex="0"
 		>
-			{#each filteredItems as item, i (i + item.value)}
+			{#each filteredItems as item, i (item.value)}
 				<div
 					data-id={item.value}
 					aria-selected={value?.value === item.value}